Bulk deleting refs is handled by Gitaly by default

Has been stable for a while in production, so this endpoint grew out if
OPT_IN, the default state.

Part of https://gitlab.com/gitlab-org/gitaly/issues/526

changelogs/unreleased/zj-opt-out-delete-refs.yml

+---
+title: Bulk deleting refs is handled by Gitaly by default
+merge_request:
+author:
+type: performance

lib/gitlab/git/repository.rb

@@ -885,7 +885,8 @@ module Gitlab
       end
 
       def delete_refs(*ref_names)
-        gitaly_migrate(:delete_refs) do |is_enabled|
+        gitaly_migrate(:delete_refs,
+                      status: Gitlab::GitalyClient::MigrationStatus::OPT_OUT) do |is_enabled|
           if is_enabled
             gitaly_delete_refs(*ref_names)
           else